Method of treating an alloy steel for enameling

ABSTRACT

A METHOD OF TREATING A COLD ROLLED SHEET OF LOW CARBON STEEL CONTAINING TITANIUM IN ORDER TO PREVENT THE OCCURRENCE OF OBJECTIONABLE SHADOW LINES NORMALLY APPEARING ON THE SURFACE THEREOF AFTER APPLYING A SINGLE OR MULTIPLE COATING OF A VITREOUS ENAMEL WHICH COMPRISES HEAT TREATING THE COLD ROLLED SHEET IN A REDUCING NON-OXIDIZING ATMOSPHERE CONTAINING CARBON MONOXIDE GAS HAVING A CONCENTRATION BETWEEN ABOUT 0.50% AND 1.50% BY VOLUME AND WITH A CONTROLLED AMOUNT OF MOISTURE SO THAT THE TREATINGH ATMOSPHERE REMAINS NON-OXIDIZING TO THE STEEL AT HEAT TREATING TEMPERATURE. THE SHEET IS TREATED IN EITHER A CONTINUOUS NORMALIZING APPARATUS OR IN AN OPENCOIL ANNEALING APPARATUS. THE REQUIRED AMOUNT OF CARBON MONOXIDE GAS CAN BE PROVIDED IN THE TREATING ATMOSPHERE AND THE TREATING ATMOSPHERE MAINTAINED NON-OXIDIZING TO THE STEEL BY PASSING AT LEAST A PORTION OF A REDUCING NONOXIDIZING TREATING ATMOSPHERE COMPRISED OF A MIXTURE OF HYDROGEN AND NITROGEN WITH A SMALL AMOUNT OF MOISTURE THROUGH A POROUS BED OF CHARCOAL DISPOSED WITHIN THE TREATING ZONE.

June 6, 1972 J. P. NOVAK 3,668,022

METHOD OF TREATING AN ALLOY STEEL FOR ENAMELING Filed 001;. 26, 197011": "I, 1 u -c 2 47 1 i i I 47' 72 l I l l I I g I l 55 T 75 TY smug:4s"

71 7O Inventor Jo kn RNova k 5 fiJM N gw fl-i-tornegps United StatesPatent 3,668,022 METHOD OF TREATING AN ALLOY STEEL FOR ENAMELING John P.Novak, Homewood, Ill., assignor to Inland Steel Company, Chicago, Ill.

Continuation-impart of application Ser. No. 686,618, Nov. 29, 1967. Thisapplication Oct. 26, 1970, Ser. No. 83,946

. rm. c1. czia 1/74 US. Cl. 148-16 7 Claims ABSTRACT OF THE DISCLOSURE Amethod of treating a cold rolled sheet of low carbon steel containingtitanium in order to prevent the occurrence of objectionable shadowlines normally appearing on the surface thereof after applying a singleor multiple coating of a vitreous enamel which comprises heat treatingthe cold rolled sheet in a reducing non-oxidizing atmosphere containingcarbon monoxide gas having a concentration between about 0.50% and 1.50%by volume and with a controlled amount of moisture so that the treatingatmosphere remains non-oxidizing to the steel at heat treatingtemperature; The sheet is treated in either a continuous normalizingapparatus or in an opencoil annealing apparatus. The required amount ofcarbon monoxide gas can be provided in the treating atmosphere and thetreating atmosphere maintained non-oxidizing to the steel by passing atleast a portion of a reducing nonoxidizing treating atmosphere comprisedof a mixture of hydrogen and nitrogen with a small amount of moisturethrough a porous bed of charcoal disposed within the treating zone.

low-carbon steel sheet material, could be coated with a single coat oflight-colored vitreous enamel without using a conventional dark-coloredground coat on the steel by incorporating in the steel melt a quantityof titanium, preferably in an amount sufiicient to combine with all ofthe uncombined carbon, nitrogen and oxygen in the steel melt (seeComstock et al. No. 2,495,835). The foregoing cold reduced-low carbonsteel sheet material containing titanium, after coating with a singlecoat of light-colored vitreous enamel, is relatively free of blistering,specking and reboiling defects commonly found in single coat whiteenameled articles of conventional steel where no dark-colored groundcoat is used. However, a serious defect is encountered as a result ofincorporating the titanium in the steel melt; namely, longitudinal linesWhich are called shadow lines or steel lines are formed on the coldreduced article or steel strip and are clearly visible after applying acoating of light vitreous enamel.

In the Bernick et al. Pat. No. 3,303,064 a method is disclosed wherebythe objectionable shadow lines can be reduced by subjecting a coldrolled titanium-containing steel sheet material to a combination oftreating steps comprising (1) a controlled heat treatment in a reducingnon-oxidizing atmosphere following the cold reduction tered in burningofi rolling lubricants, and (2) pickling the heat treated steel in anacid pickling bath. of any conventional composition to effect a pickleweight loss between about .60 and 2.2 grams per square foot of surfacearea.-The cold reduced steel treated in the foregoing manner can then besubjected to a conventional single coat of light-colored vitreousenameling treatment which preferably includes a conventional nickelflash coating prior to enameling to improve enamel adherence inaccordance with good enameling practice.

In the practice of the foregoing method of Pat. No. 3,303,064, however,it has been found that the desired elimination of the shadow lines isnot consistently obtained and that the enamel adherence properties ofthe treated sheet steel material could be improved.

Accordingly, it is an object of the present invention to provide animproved and more economical method of treating a cold reducedlow-carbon titanium bearing steel sheet material which makes the steelsheet material better adapted to being coated with a single coatoflight-colored or white vitreous enamel without exhibiting objectionableshadow lines.

It is a further object of the present'invention to provide a method oftreating a cold reduced low-carbon titanium bearing steel sheet materialwhich consistently removes objectionable shadow lines and improves theenamel adherence properties of the steel sheet material.

Other objects of the present invention will be apparent to those skilledin the art when the following detailed description is read inconjunction with the accompanying drawing wherein:

FIG. 1 is a vertical sectional view of an open coil annealing furnacesuitable for use in the present invention; and

FIG. 2 is a schematic vertical sectional view partially in elevation ofa modified form of open coil annealing apparatus suitable for use in thepresent invention.

The foregoing objects of the invention can be achieved by subjecting acold reduced titanium bearing steel sheet material which normallyexhibits objectionable shadow lines to a controlled treatment at anelevated temperature in a reducing atmosphere which is non-oxidizing tothe steel and which contains a small amount of carbon monoxide gas. Theinclusion of carbon monoxide gas in a concentration of between about.50'% and 1.50% by volume in the reducing non-oxidizing treatingatmosphere accelerates the removal of the shadow lines from the coldreduced titanium-containing steel sheet material and improves the enameladherence properties of the treated material. The carbon monoxide gasappears to act as a catalyst in effecting the removal of the shadowlines. Any suitable manner or means can be employed for providing-thecarbon monoxide gas in the treating atmos- .phere.

' It is essential that the moisture content of the treating atmosphere,be carefully controlled to avoid excess water, since the ratio ofhydrogen to water in the treating atmosphere comprised essentially ofhydrogen and nitrogen at a particular treating temperature largelydetermines whether the treating atmosphere has a reducing or oxidiz- 1ingaction on the surface of the steel strip, and this ratio thereof andbefore the cold reduced steel has been exposed to a highly oxidizingatmosphere, such as encounvaries with the treating temperature. Duringthe initial or heating up period when the steel temperature isrelatively low, the dew point of the treating atmosphere is kept 10W inthe range of about 30 F. and is preferably maintained not greater thanabout -4 F. After the heating up period, the dew point of the treatingatmosphere can vary as high as +40 F. or above, provided the temperatureis sufficiently elevated that the treating atmosphere remainsnon-oxidizing to the steel and preferably remains reducing. In thepreferred process, the maximum moisture content of the treatingatmosphere measured at the point of withdrawal from the treating chamberis betweena .dew .pint of about 09.1 and a. maximum .ofl

about -+35 F. with the atmosphere preferably being maintained at a dewpoint of about F. as measured atrthez point of withdrawal from thetreating chamber. The manner of controlling the moisture content of theinput treating atmosphere gas is not critical and any means canbeemployed which permits maintaining a relatively low dew point .so thatthe atmosphere remains non-oxidizing to the steel atthe temperature towhich the steel is heated. i

The reducing atmosphere is preferably formed of HNX "gas which iscomprised of a mixture of betweenabout 4% to 14% hydrogen and thebalance being nitrogen with a small amount of water. Other commonreducing nonoxidizing gaseousi atmospheres which can be used in thepresent. invention are DX gas and disassociated ammonia which comprisesabout 75% hydrogen and 25% nitrogen. It has been found that a verysatisfactory method of supplying the required amount of carbon monoxidegas in a relatively dry. reducing non-oxidizing treating atmospherecomprised mainly of hydrogen and nitrogen and forsimultaneouslycontrolling the water vapor content sothatthe treating atmosphereremains non-oxidizing to the steel is to pass the reducing gas through aporous bed of charcoal heated to an elevated temperature of betweenabout 1600" F. and ,1950" F., and preferably having a temperaturebetween about 1600 F. and 1700" F. When a treating. atmosphere comprisedmainly of hydrogen and nitrogen with a small amount of moisture ispassed through a porous bed of charcoal heated to an annealingtemperature, the moisture in the treating gas reacts with the charcoalto produce an amount of carbon monoxide within the range of about 0.50%and 1.50% by volume which effects rapid removal of shadow lines from thesheet material. The carbon bed reaction simultaneously removes from thetreating atmosphere 'a portion of the water which tends to build up whenthe at- .mosphere is recycled in the treating zone. The amount ofcharcoal required for effecting the foregoing reaction is about twopounds per ton of steel being treated with a minimum quantity of aboutten pounds for treating the usual open coil in a conventional open coilannealing furnace. In each instance the quantity of charcoal used in thebed is that required to insure intimate contact between the treatingatmosphere and carbon. It should be understood, however, that the entirequantity of charcoal in the bed is not converted into carbon monoxideduring the heat treatment process.

If preferred, the water vapor which tends to accumulate when thetreating atmosphere is recycled can be removed by a treatment other thanthat by which the carbon monoxide gas is provided in the treatingatmosphere. For example, water can be removed from the treatingatmosphere by any known physical process, such as by absorption,adsorption, transformation, or chilling to efiect precipitation, or byany chemical means. The carbon monoxide gas can also be provided byintroducing carbon monoxide gas directly into the treating chamber orzone from a suitable supply source or by utilizing any other knownchemical reactions to provide carbon monoxide gas in the reducingtreating atmosphere without depart- :ing from the scope of the presentinvention.

As an alternate method of providing the required amount of carbonmonoxide in a dry HNX treating atmosphere without causing the atmosphereto become oxidizing to the steel at the treating temperature, carbondioxide gas in a concentration ranging between about 0.50%

to 1.50% by volume can be added to the treating atmos- F., and the dewpoint of the treating atmosphere should not exceed-a maximunrvalue ofabout 409 F. at the point of withdrawal from the treating zone so as toprevent the treating atmosphere oxidizing the surface of the steel.

The temperature and time of heating used in the treating step of thepresent invention can be varied 'considerably without detracting fromthe improved results. Genorally," the temperature and time of heattreating the cold reduced low carbon titanium-containing steel in aclosed chamber are adjusted to provide the steel with the desiredmechanical properties. For example, an enameling steel sheet materialmust have the necessary strain and aging characteristics for deepdrawing, and the heat treating temperature is adjusted accordingly. Inevery instance, however, the atmosphere in the heating chamber iscarefully controlled so that the atmosphere in which the steel is heatedremains reducing during the heat treating period and doesnot becomeoxidizing to the steel while the exposed surface of the strip orsheet-is, at an elevated temperature.

The required treatment of a cold reduced titaniumcontaining steel stripcan be continuouslycarried out by employing a continuous normalizingfurnace or con- :tinuous annealing furnace wherein the temperature iskept within the herein specified limits and the treating atmosphere ismaintained in a reducing non-oxidizing state, or can also be performedbatchwise in an open'coil annealing furnace. Accordingly, whencontinuously heat treat ing a steel strip in a continuous normalizer ofconventional design, the steel strip is heated continuously by passingthrough a heated zone having a reducing nonoxidizing atmosphere whichcontains between about 0.5% and 1.5% by volume CO with the dew point ofthe treating atmosphere before contacting the steel strip preferablyranging between about 30" F. and --10 F. and wherein the steel strip isheated to a temperature of at least 1000 F. for a controlled period oftime. For example, the strip can be treated for about two minutes in thecontinuous normalizer when the temperature is about 1650 F. to effectremoval of the shadow 'lines. a a The desired-beneficial effects on theenameling properties and mechanical properties of the steel can also beachieved by heat treating the cold reduced titaniumbe'aring steel sheetmaterial'batchwise at a temperature within the temperature range hereinspecified in an open coil annealing'furnace. In the open coil annealingtreatment, the steel strip is placed in an open coil form to prov despaces between the laps of the coil for the passage of gas over thesurface of the strip which'is slowly heatedto "a temperature of about1620 F. over a period of between 2 and 4 hours, is held at thetemperature of about 1620 F. fora period of between 2 and 3 hours, andis then heated to a temperature of between about .1675? F. to 1695 F.and held atthe latter temperature for between about 30 minutes and onehour. The atmosphere in the open coil annealing furnace, is preferablyHNX gas, but can be DX gas 'or disassociated ammonia, with the gasalways having about 0.5-1.5 percent 'by volume CO and a sufiiciently lowdew point toremain non-oxidizing to the steel at the temperature of thegas in contact with the steel. The dew point of the HNX gas asintroduced into the treating zone at the beginning of the' treatingcycle when the temperature of the steel may be relatively low can be aslow as 40 F. and is preferably not greater than about 4 F. The dew pointof the treating atmosphere as withdrawn from the treating chamber whenthe temperature of the'steel is relatively high'can reach'abou t +35 -F.Without being oxidizing to the steel. e a Following the heat treatmentof the cold reduced titanium-bearing steel strip, the strip must bepickled by immersingin'an acid pickling bath until a critical pickleweight loss iseffected in order to obtain a satisfactory single coatlight-colored vitreous enamel product. Thus, ithas been found necessaryto maintain the heat treated steel strip or article in an acid picklingbath, such'as an aqueous sulfuric acid solution containing 6%10% byweight sulfuric acid, until the pickle loss is at least about .60 gramper square foot and not greater than about 2.2 grams per square foot.The preferred amount of pickle loss effected ranges between .75 gram persquare foot and 2.0 grams per square foot. Within the latter range ofpickle loss the single coat light-colored vitreous enamel strip exhibitsexcellent enamel adherence and is free of shadow lines.

In the practicing of the improved process of the present invention fortreating a coil of titanium-bearing coldrolled steel sheet material, thematerial in the form of an open coil is placed in an open coil annealingapparatus of the type shown in FIG. 1 of the drawing, wherein thefoundation 40 supports a furnace base 45 which in turn supports an outerfurnace bell 46 and an inner cover 47 which may be of any suitabledesign and typeJA' centrifugal fan or blower 51 is supported onavertical shaft 52 which projects down through the bottom portion53 ofthe base. The fan or blower 51 can be driven by any suitable means, suchas the belt 541'- A diffuser 55 surrounds the fan 51 and directs thedischarge there of outwardly and upwardly through the annular opening 56at the rim of the diffuser. Included in the diffuser 55 are a pluralityof radially extending vanes 57, the upper edges of which lie in a commonhorizontal plane.

The plenum chamber and charge support P is supported on the top edges ofthe vanes 57 of the difiiuser 55 and includes an annular base plate 58having a plurality of circumferentially spaced radially extendingvertical webs 59 secured thereto. The upper edges of webs 59 all lie ina common horizontal plane and serve to engage and support the open coilC disposed thereon. An annular baflie wall 60 has its outer edge securedto the cylindrical outer Wall 61 of the plenum chamber P and is inclinedinwardly and downwardly so that its inner periphery is secured to theannular base plate 58 at the center opening therein. Bathe wall 60, inaddition to directing the atmosphere which has been forced downwardlythrough the spaces between the laps of the coil C into the inlet of fan51, reinforcesthe vertical webs 59 and assists in giving the necessaryrigidity to the entire plenum upper open end of the axial passage at thecenterof the lower section of the inner chamber of the annealingapparatus and conveying the treating atmosphere to a separatecarbonaceous gas mixing and demoisturizer chamber 72- in whichadditional carbon monoxide gas is mixed withthe treating atmosphere andthe dew point of the atmosphere lowered to an acceptable level. Thetreating gasor" atmosphere from chamber 72 is then conveyed throughconduit 75 to open coil annealing apparatus where it enters the innerchamber thereof and again comes in-contact with the surface of the opencoiled steel strip being treated. While the preferred method of addingtherequired amount of carbon monoxide gas and control- ,ling themoisture content of the treating atmosphere within the chamber 72 is bypassing the atmosphere through a bed of heated charcoal disposed inchamber 72, it is possible to inject into chamber 72 the requiredquantity of carbon monoxide gas or another carbonaceous gas, such ascarbon dioxide or a hydrocarbon gas, which will react with a.compone ntof the treating atmosphere, such as hydrogen or water vapor, at thetemperature of the treating chamber or which will be decomposed at theelevated treating temperature to provide at equilibrium a suitablequantity of carbon monoxide gas. And, moisture can be removed from thetreating atmosphere within chamber 72 by reacting the moisture in thetreating atmosphere with a suitable chemical reagent or by knownphysical treatments for removing moisture before returning theatmosphere to the open coil annealing apparatus. The secondary treatingchamber 72 as shown in FIG. 2 can be adapted for use in combination witha continuous normalizing or continuous annealing furnace for maintainingthe treating atmosphere thereof in the required reduced no-oxidizingcondition for continuous treatment of an endless strip of titaniumbearing steel, if desired.

A still further modified form of the present invention in which thereducing non-oxidizing treating atmosphere containing carbon monoxidegas is used can be carried out in a conventional open coil annealingfurnace by "double-stacking an open coil of high carbon steel a-bove theopen coil of low carbon titanium steel which is to be treated to preventformation of shadow lines. The open coil of carbon steel double-stackedwith the open coil of -.t itanium. bearing steel contains a sufiicientquantity of of carbon monoxide gas and simultaneously efiect's loweringthe "moisture content thereof to the required low levels.

The relatively high carbon content of the carbon steel is coil C. Ifdesired a special charcoal chamber can be mounted adjacent the opening56 for retaining a-- loose bed of porous charcoal material throughwhich'all 'of the gas being recycled passes before'contacting thesurface of'the open coil. 7 v

A modified form of open coil annealing apparatus which can be used inthe present invention is shown in FIG. 2 of the drawing, wherein atleast a portion of the treating atmosphere of the open coil annealingchamber, after passing over the surface of the steel strip beingtreated, is

preferably continuously.withdrawnfrom the open coil the moisture contentof the treatingatmosphere is adjusted so as to provide the required dewpoint in' the treating chamber. In FIG. 2 the open coil annealingapparatus parts correspond with the similarly numbered parts describedin FIG. 1, and differ therefrom by having a gas outlet conduit 70 whichhas a blower 71 associated therewith for drawingthe treatingatmo'spherefrom the equivalent to having a quantity of coke within the open coilannealing apparatus.

In treating the steel sheet material with open coil annealing';apparatus in accordance with the present invention it will be understoodthat it is advisable to employ good opencoil annealing practices inorder to obtain the desired results. For example, it is important tocorrelate the lap gap between the coil winding and the rate of flow ofthe treating atmosphere in order to insure adequate gas-flow over thesurface of the sheet material and to avoid pockets of high moisturecontent. Thus, if the lap gap is narrow, a higher gas flow rate isrequired. Also, the vv idth of the coil has an important effect on theflow of gas, with the wider coils requiring a wider lap gap or a highergas flow rate to provide proper treatment thereof. In coils having aWidth of 30 to 40 inches and above a single wire spacer between the coillaps is generally inadequate, and it is preferable to use multiplespacer wires to promote a more uniform gas flow between the laps of thecoil. A treating gas flow rate of at least about 1200 cubic feet perhour of HNX gas is used. Higher gas flow rates in excess of 2000 cubicfeet per hour can also be used and are preferred for all coils exceptthe very heavy gauge narrow coils.

The following specific example is for the purpose of furtherillustrating a treatment of steel sheet material in invention.

accordance with the preferred process of the present EXAMPLE I Coils oflow carbon titanium bearing steel having a gauge of .0359 inch andhaving the following approximate composition: .04% C, 35% 'Mn, 34%.T;-'.048% Al and .03 Cu with the remainder being essentially iron, wererewound as open coils using two-.075 inch wires twisted on a .075 inchwire to provide a lap gap of .225 inches. The coils were single stacked(or double stacked where indicated) on-an open coil annealing stand ofthe type shown in FIG. 1, with an open mesh tray containing about-20pounds of porous charcoal briquettes supported on the exposed upper endof. a coil being heat treated within the inner cover of the annealingapparatus. At equilibrium, the recirculating treating atmospherecomprised of HNX gas H and 90% N). cohtainsbetween 0.5% and 1.5% byvolume carbon monoxide. The temperature within the apparatus and thetreating atmosphere was regulated to provide the following heat treat- 1ment cycle and operating conditions: 1

(1) Temperature of heat treating cycle.

(a) 1 620/1640 F./6 hour soak, single stack (12 hour soak when doublestacked).

(b) 1660/1680 1 1/2 hour soak.

(2) AtmOsphere-HNX gas. f

(a) Flow rate2000 cfh. v p (b) Dew point: Inflowing gas, 30 F; tit-10F.Dew point: Exhaust readings, ;+9 F. to +351 F.

Following the foregoing heat treatment cycle the coils were pickled in a9% by wt. aqueous sulfuric acid bath at a temperature of 170 F. for aperiod of 5 minutes and thereafter coated in a conventional manner Withasingle coat of light-colored vitreous enamel.

The porcelain enamel test results of coils processed in the foregoingmanner are shown in the following Table I:

8 (a) disposing in a treating zone a cold reduced titanium-containingsteel sheet material having an amount of titanium at least sufiicient tocombine with all the carbon in said steelsheetmaterial and whose surfacehas not been subjected to an oxidizing treatment after cold reduction,

(b) heating said steel sheet material in said treating zone to atemperature between about 1600 F. and

195N1 2, and i A (e) maintaining in intimate contact with said heatedsteel sheetmaterial in said treating zone a reducing atmosphere which isnon-oxidizing" to' said' steel sheet material consisting essentially ofhydrogenand.. ni-

trog en with suificient (water vapor to ielfecfadw point of about F. orless during the initial heat up periodand whichpcontains a concentrationof between about 0.50% and 1.50% by volume of carbon monoxide gas.' 7 j2. A method as in claim 1, wherein said carbon monoxide is formed bybringinga said reducing atmosphere having a maximum dew'point of aboutF. into intimate contact with a bed of porous charcoal at a temperaturebetween about 1600F. and 1950 F.

3. A method as in claim 2', wherein said bed of porous charcoal isdisposed within said treating zone containing said steel sheet material.I v

4. A method as in claim 2, wherein said treating zone consists of, anopen-coil annealing chamber, and said atmosphere is cycled in contactwith a coil of said steel sheet material at a temperature between about1600 F. and 170.0: F. for a period of about-.8 hours; whereby said coilis simultaneously heat, treated while removal of shadow linesiseffected. V I

5. A method as in claim 1, wherein said atmosphere is passed through aregenerating zone spaced from said treating zone containing said steelsheet material and carbon dioxide gas is supplied to said atmosphere andTABLE I.SUMMARY 0F PoRCELAlNvgfiNAMEL TEST RESULTS OF TI-NAMEL COILSPROCESSED THROUGH THE 00A ITH AND WITHOUT CHARCOAL BBIQUETS Pickle loss,grn./sq.'t't., 5 Appearance, shadow line rating, min. in 170 F., 9%H2804 Adherence, percent of tests percent of tests N 0. of Aver- No. ofExcel- No. of Very Modermentity tests agev Range tests lent Good FanPoor tests None lite Lite ate 40lcoils; processed as in Example 7 "14single stacked 2s 1. 66 0. 77 2.40 56 20 double stacked 56 1. 46 0.63-3. 44 283 83 11 3 3 144 20 coils, processed as in Example I g 1: 20single stacked 70 1. 25 0.66-2.00 410 73 I 19 7 1 205 40 coils,processed as in Example 1, without charcoal: 19 and 20 single stacked80 1. 01 0. 60-2. 01 v t 478 4 8 33 239 Y 61 I 20 17 2 Open CoilAnnealer.

It .will be evident from the foregoing data th at substantialimprovements were obtained where the treating gas is passed through abed of porous charcoal as compared with the results obtained when anidentical .treating cycle was employed but without passing the treatinggases through a bed of porous charcoal.

In each of the embodiments of the present invention in which thetreating atmosphere is brought into; contact with a source of solidcarbon, the solid carbon performs the dual functions in the reducingnon-oxidizing atmosphere of reducing the amount of moisture in thetreating atmosphere to a relatively low level so that the treatingatmosphere remains non-oxidizing to the steel and providing a quantityof carbon monoxide gas for the reducingnonoxidizing treating atmosphereso as to readily etfect'removal of the material in the steel which isresponsible for the formation of objectionable shadow 'lines'and withoutadversely effecting the surface or internal properties of the steel. Iclaim: I 1. In a method of removing objectionable shadow lines from coldreduced titanium-containing steel sheet mate"- rial, the improvementcomprising;

moisture removed from said atmosphere in said regenerating zone. andthereafter returning said atmosphere to said treating zone. V

6; A method as in claim 1,wherein said treating zone containing said'sheet material is a continuous normalizing chamber and wherein saidatmosphere containing said carbon monoxide gas is brought into' contactwith said steel sheet material at a temperature of about 1650f F.

for a period of about 2 minutes.

7. method as claim 1, wherein said carbon monoxide gas content of saidatmosphere 'isp'rovided by adding a hydrocarbon gas to said atmosphere.

